Method and pre-flux for coating ferrous metals with nickel prior to galvanizing



Sept. 14, 1965 J. R. DAESEN 3,206,324

METHOD AND FIRE-FLUX FOR COATING FERROUS METALS WITH NICKEL PRIOR TOGALVANIZING Filed June 22,' 1961 FERROUS METAL TO BE GALVANIZED SURFACECLEANING TREATMENT TREATMENT WITH AQUEOUS SOLUTION CONTAINING ALKALIHALIDE SALT, NICKEL CHLORIDE AND A SUBSTANCE CONTAINING A FREE ALDEHYDEGROUP INTRODUCTION OF TREATED METAL INTO GALVANIZING BATH WITHDRAWAL OFGALVANIZED METAL FROM GALVANIZING BATH IN VENTOR.

JOHN R. DAESEN ATTORNEYS United States Patent 3 206,324 METHOD ANDPRE-hLUX FOR COATING FER- ROUS METALS WITH NICKEL PRIOR TO GAL- VANIZINGJohn R. Daesen, 131 E. Cuttriss Place, Park Ridge, Ill. Filed June 22,1961, Ser. No. 118,784 14 Claims. (Cl. 117-51) This invention relates tothe preparation of ferrous metals for the operation of coating with zincor zinc alloys by immersion in molten baths of the coating metal,commonly called hot dip galvanizing, and to a novel flux which enables athin layer of reduced nickel to be deposited on ferrous metals.

Hot dip galvanized coatings consist essentially of a layer or layers ofzinc-iron alloy next to a ferrous base and an outer layer which issubstantially the composition of the molten bath in which the steel basehas been immersed.

The relative amounts and the distribution of the various layers ofdiffering composition have a profound effect on the physical propertiesof the coating, including ductility and smoothness of coating, and adefinite, though less universally recognized, effect on the chemicalcharacteristics of the coating, specifically with reference toresistance to corrosion. Accordingly, many means have been developed tocontrol the relative amounts and distribution of these differentcomponents of the coating.

Most of the hot dip galvanized steel sheet and strip now produced iscoated in a molten bath of zinc to which suflicient aluminum has beenadded (.07 to .15 percent) to restrain the formation of zinc-iron layersto the extent that they form only a few percent of the total coating.

In the galvanizing of metal with baths containing insufficient aluminumto greatly restrict alloying (.02 percent aluminum or less), commonlycalled conventional hot-dip galvanizing, it has been recognized that amore moderate control of the development of the zinc-iron layers duringthe coating operation may be secured by a number of means including lowtemperature of coating bath, use of steel low in carbon, silicon andphosphorus, annealing the steel base before galvanizing, avoidance ofsevere pickling which leaves a smut of metallic ferrous particles on thesurface, thorough rinsing of all ferrous salts from the base beforeentrance to the molten bath, and cooling the work promptly as it iswithdrawn from the molten coating bath.

Many liquid solutions, called pre-fiuxes, some of them proprietary, havebeen used to coat the ferrous base material (after cleaning or picklingand before dipping in the molten zinc) with a layer of salts, usuallyzinc and ammonium chlorides. These solutions have a dual function whichto some extent is self contradictory. During the time of exposure of thepre-fiuxed steel to the air, these solutions are intended topreventdissolution to or re-oxidation of the cleaned steel surface, whileduring the immersion of the steel in the molten bath, the prefluxmaterial reacts with any zinc or iron oxide formed on the bath or on thework to permit clean steel to make contact with clean zinc. Ineviablyduring atmospheric exposure and entrance into the molten bath, thepre-flux solutions also attack to some extent the clean steel of thebase thereby contributing in varying degree to an increased amount ofzinc-iron alloy in the coating and often to a proliferation of itirregularly towards the surface of the coating.

Dissolution of the steel base occurs in the floating cover of frothyflux, which is maintained (except in so-called dry galvanizing) on theentrance surface of the zinc bath.

The art of practical hot dip galvanizing consists largely 3,205,324Patented Sept. 14, 1965 "ice in the selection of materials for and themanipulation of the Work through these preparatory pre-fiux baths andmolten flux covers to achieve, for the particular product in each case,the needed cleaning action without undue dissolution of the steel base;for this results in uneconomically high percentages of zinc-iron drossin the bath, or unfavorable amounts and distribution of zinciron alloysin the coating.

If the steel base is coated with nickel before being immersed in moltenzinc, it not only will be protected from oxidation during the intervalbetween pickling and dipping in molten zinc, but its reaction rate withthe zinc bath, and the growth of the various alloy layers by diffusionwill be so reduced that a substantial and useful improvement in surfaceappearance, physical properties and corrosion resistance of the coatingwill result. In addition, the production of zinc-iron dross in the bathwill be materially reduced.

This advantageous effect is secured even though the amount of nickeldeposited be so small that none of it remains in unreacted condition onthe steel base after the immersion in molten zinc. This improved coatingis therefore not of the type described by Schoonmaker et al. in US.Patent 2,315,740, etc., where the outer zinc coating acted as a sealerof pits or imperfections in an underlying nickel coat.

Nickel may be applied to a steel base by electro-deposition, by chemicaldeposition without the use of electric current as inhypophosphite-containing baths or the cyanide-containing so-called blacknickel solution with or without the use of an externally connectedelectrode.

Unalloyed nickel deposits are superior for the purpose to thosecontaining phosphorus or sulphur, especially when the nickel deposit isthin and of course only thin nickel coatings, of the order of of a milthick are economically feasible when commercial operations areconsidered.

Regardless of the extent of prior recognition, acceptance or disclosureof any of the above considerations, and the limitation these may be heldto impose on the novelty of my discovery, I have found by considerableexperimentation a new means of application of metallic nickel to a steelbase, never before disclosed or known to my knowledge, that has suchadvantages in economy of operation and investment in material andequipment that it makes commercially attractive for the first time, thecontrol of the reaction between zinc and iron in the hot dip galvanizingoperation through the use of nickel coating so as to produce coatingsthat are superior in appear anec, physical properties and resistance tocorrosion in atmospheric exposure to coatings not so treated. These andother objects are achieved according to the present invention byproviding a pre-flux solution comprising nickelous chloride, an alkalimetal halide, and a substance containing a free aldehydic group for usein a galvanizing process.

The use of the easily oxidized aldehydes for the reduction of metalsfrom aqueous solutions of their salts, depositing them on metallic ornon-metallic surfaces has long been known. Examples of such knownprocess include the production of silver mirrors by the action of invertsugar on silver nitrate solutions, or the formation of copper layersfrom the action of formaldehyde on cuprotartrate solution in thepresence of silver. (Misciattelli, US. Patent 2,183,202.) Typical amongthe substances which contain a free aldehydic group and which may beused in accordance with the present invention are aldehydes, aldoses,and carbohydrates.

Precipitation of nickel from nickel chloride solutions by sugar orglucose is an effective means of depositing sufficient nickel for theaccomplishment of this improvement on a steel base, without the use ofelectric current or an externally connected electrode to provide anelectrolytic potential. Prior to my invention, however, it has not beenpossible to secure preparatory coatings of nickel that were satisfactoryfor the control of the reaction between the steel base and the moltenzinc in hot dip galvanizing except in highly concentrated nickelchloride solutions of 35 percent or more by Weight The excessive cost oflarge tanks (commercial installations may hold 3,000 to 6,000 gallonsfor the immersion of large articles) of such an expensive solution iscommercially prohibitive, considering risk of loss by leakage, polutionand carry-over as well as interest on investment.

A great many attempts to discover a solution effective for thedeposition of the required amount and quality of nickel, while avoidingthe excessive cost of solutions high in nickel chloride content havebeen made, but many of the manipulations that suggest themselves werefound ineffective when tried.

Incorporation of nickel chloride in solutions of the usual zinc-ammoniumchloride used as a pre-dip flux failed to yield the desired results.Additions of aluminum chloride to nickel chloride solutions ofdecreasing concentration inhibited the deposition of nickel. The use ofboric acid, citric acid or tartaric acid was found ineffectual forsecuring the desired result.

Manipulation of the hydrogen ion concentration of lower concentrationnickel chloride solution form a pH of 3 to a pH of 8 did not result insatisfactory deposits.

Reasoning that since other manipulations did not result in a deposit ofthe desired coating, the near saturation, with respect to nickelchloride, of the high concentration solutions might be the cause ofready deposition of the nickel, trials were made of solutions in whichone half or three quarters of the nickel chloride hexahydrate werereplaced by like amounts of highly soluble salts of alkali halides. Theresults were successful. A

It was later found that successful coatings were deposited in solutionscontaining glucose as a reducing agent with nickel chlorideconcentrations as low as 5 percent by weight of nickelous chloridehexahydrates, even when the alkali halide content was reduced to aslittle as 5 percent by Weight. Lower percentages of nickelous chloridehexahydrate or alkali halides were not investigated since the exactpoint between and percent at which the effect ceases would not be ofgreat practical importance, but the recitation of 5 percent is of coursemeant to include the smal range below 5 percent in which the effect isoperative.

The alkali halide, sodium and potassium chloride in normal solution haveelectrical resistivities of 11.6 and 8.94 ohms cm. respectively. Theseare greater than the resistivity of mineral acids and alkali hydroxides,neither of which are useful for depositing nickel for the end desired.It is believed that these acids and bases would be effective if otherprocesses such as acid attack of the ferrous metal did not occur. Withthese exceptions however, the alkali halides are lower in electricalresistivity in normal solution than all other commercially useful salts.

Whether the success of the use of alkali halides in solutions containinglow concentrations of nickel chlorides and a reducing agent is due tothe improvement in electrical conductivity they confer to the solution(which may be important in small local region of deposition even thoughno electrical current is imposed as in electroplating), or due to makingthe solution more nearly saturated for the nickel ions by the additionof compounds containing more negative metals combined with the commonchloride ion or some other cause is not known. Several eifects may beoperating.

Be sure that as it may, I have discovered that the failure oflowconcentration solutions of nickel chloride to be reduced by aldehydesor substances containing an aldehydic group ;;(one oxygen and, onehydrogen atom at- ,7 5 percent water tached to a carbon atom), can beremedied at low cost by the use of an inexpensive, highly soluble alkalihalide salt; sodium chloride is eminently suitable for this purpose. Thesolutions are effective from normal room temperature to 200 degrees F.In the lower part of the range, deposition is slower. At temperaturesover degrees F., consideration must be given to evaporation of thesolution.

An example of my invention is as follows. A piece of hot rolled, lowcarbon steel sheet, .100 thick was pickled in an 8 percent solution ofsulfuric acid for 15 minutes at F., rinsed in running water, andimmersed for 30 minutes (at 150 F.) in a solution containing, by weight:

5 percent nickel chloride hexahydrate 10 percent sodium chloride 10 percent glucose 75 percent water On removal it was allowed to drain for 5minutes and was then immersed promptly through a layer of zincammoniumchloride flux cover into a bath of molten zinc at 855 F. for a period oftwo minutes. It was then withdrawn through a surface uncovered by fluxand cooled in the air.

Visual examination showed the outer surface of the galvanized sheet tobe bright and smooth, and examination of the structure of the coatingunder the microscope showed it to consist of dense compact alloy layersfor about one-third of the thickness adjoining the steel base, while theouter two-thirds of the thickness of the coating was zinc of thecomposition of the galvanizing bath, with no proliferation or branchingout of the outer alloy layers irregularly into the outer zinc layer, asis commonly found in conventional hot dip galvanized coatings (aluminumless than .03 percent) produced without the restraining effect of anickel pre-coat.

The resistivity of the pro-treating solution of this example measured atroom temperature was found to be about 8 ohms cm. A solution containing5 percent by weight of nickel chloride hexahydrate in water was found tohave an electrical resistivity, on the other hand, of about 26 ohms cm.;and a 10 percent solution had a resistivity of 17 ohms cm.

In a second example, similar procedure and hot dip galvanizing were usedwith a pro-treatment bath, following pickling and rinsing, of thefollowing composition by weight:

5 percent nickel chloride hexahydrate 5 percent sodium chloride 30percent glucose 60 percent water Similar results were obtained as in thefirst example. The use of the higher content of glucose appears to havean advantage in providing an oxidation resisting film, when there is adelay in immersion of the pre-treated article in the molten zinc bath,but glucose contents as low as 10 percent give satisfactory results inthe above solution.

The electrical resistivity of the pre-treat-ing solution of the secondexample, measured at room temperature, was found to be about 12 ohms cm.This is substantially the resistivity of a solution containing 40percent nickel chloride hexahydrate, 20 percent glucose and 40 percentwater by weight, which was also effective in producing a satisfactorynickel coating for securing the described benefits on subsequentgalvanizing.

In a third example, after customary pickling and rinsing, a sheet of hotrolled, low carbon sheet steel .100 thick was immersed for 20 minutes(at 180 F.) in a solution containing, by weight:

10 percent nickelous chloride hexahydrate 5 percent potassium chloride10 percent glucose and rinsed in water. An adherent deposit of nickelwas obtained. After passing the sheet through a molten flux ofzinc-ammonium chloride into a molten zinc bath at 850 F. for two minutesand cooling in air, the surface of the coating was found to be brightand smooth, and microscopic examination of the coating in section showedit to consist of dense, compact alloy layers comprising less than halfof the thickness of coating with an outer layer of zinc of thecomposition of the galvanizing bath. There was no proliferation orbranching out of the alloy layers into the outer zinc layer.

The resistivity of the pre-treating solution of this example, measuredat room temperature, was found to be about 9 ohms cm.

In a fourth example, after customary pickling and rinsing, a piece ofhot rolled, low carbon sheet steel .100" thick was immersed for 20minutes in a solution at 180 F. containing, by weigh:

5 percent nickelous chloride hexahydrate 10 percent sodium chloride 5:percent iso-butyraldehyde 80 percent water and rinsed in water. Anadherent deposit of nickel was obtained. After rinsing, the sheet waspassed through a molten flux cover of zinc-ammonium chloride into amolten zinc bath at 850 F. for two minutes, removed through a surfaceuncovered by flux, and air cooled. The resulting coating was similar tothat secured in other examples described.

The resistivity of the pro-treating solution of this fourth example,measured at room temperature, was found to be about 7 ohms cm.

Further experiments indicated that calcium chloride, barium chloride andmagnesium chloride satisfactorily facilitate the coating of nickel on aferrous metal from solutions having less than 35 percent by weightnickelous chloride.

Other halide salts were found effective in securing the desired resultsin solutions containing 10 percent by weight of nickel chloridehexahydrate With an aldehyde in water. Some of the resistivitiesobtained are indicated below:

Addit-ion Resistivity, ohms cm. 10 percent barium chloride 10 percentcalcium chloride 10 percent magnesium chloride 10 percent ammoniumchloride 10 percent potassium bromide 10 percent potassium iodide 5percent sodium bi-fiuoride 11.

In general, the chlorides appear to be the most effective in depositionof a satisfactory pre-coat. The bromide salt was less effective butappeared superior to the iodide and the bifiuoride. This supports thebelief that deposition of nickel in the form desired is facilitated byintroduction, by means of the addition salt, of the common ion,chlorine, to the nickel chloride solution. going alkali metal (includingammonium in this sense) and alkaline earth metal halide salts,therefore, when used in an amount such as to yield aqueous solutionhaving electrical resistivity of less than 12 ohm cm. measured at roomtemperature, may be used.

Since in no case Were satisfactory nickel deposits secured with nickelchloride solutions containing less than 35 percent nickel chloridehexahydrate except where the resistivity was lowered by additions of asoluble salt yielding low resistivity solutions, it appears that the useof such means for improving electrical conductivity is a significantcontrol in the lowering of the nickel chloride The forecontent requiredfor nickel coating to values that are reasonable and commerciallyfeasible.

Having now particularly described and disclosed the nature of myinvention and the manner in which the same is to be performed, I claim:

1. A method of coating a ferrous metal with a thin coating of nickelprior to hot dip galvanizing, comprising, pickling the ferrous metal,and passing the metal through an aqueous pre-flux solution corn-prisingfrom 5 percent to 35 percent by weight of nickelous chloridehexahydrate, at least 5 percent by weight of at least one halide saltselected from the group consisting of ammonium halides, alkali metalhalides and alkaline earth meatl halides, and at least 10 percent byweight of an aldose.

2. A method of coating a ferrous metal as claimed in claim 1, in whichthe halide salt is a chloride.

3. A method of coating a ferrous metal as claimed in claim 2, in whichthe halide salt is sodium chloride.

4. A method of coating ferrous metals prior to hot dip galvanizing whichcomprises the step of passing a pickled ferrous metal through an aqueousprerflux solution containing less than 35% by weight nickelous chloride,at least 5% by weight of a substance containing a free alaldehydic groupand at least 5% by weight of at least one soluble halide salt selectedfrom the group consisting of ammonium halides, alkali metal halides andalkaline earth metal halides.

5. A method according to claim 4 wherein said halide salt is an alkalimetal chloride.

6. A method according to claim 4 wherein said halide salt is an alkalineearth metal chloride.

7. A method according to claim 4 wherein said substance containing afree aldehydic group is at least one selected from the group consistingof aldehydes, carbohydrates and aldoses.

8. A method of galvanizing a ferrous metal comprising pickling saidferrous metal, passing the pickled metal through an aqueous pre-fluxsolution comprising an aqueous solution of less than 35% by weight ofnickelous chloride hexahydrate, at least 5% by weight of a substancehaving a .free aldehydic group, and at least 5% by weight of at leastone soluble halide salt selected from the group consisting of ammoniumhalides, alkali metal halides and alkaline earth metal halides, andsubsequently passing the pre-fluxed metal through a galvanizing bath.

9. A method according to claim 8, wherein the aqueous pre-flux solutioncontains from 5 to 35% by Weight of nickelous chloride hexahydrate, from5 to 10% by weight of sodium chloride, and from 5 to 30% by Weight of asubstance having a free aldehydic group.

10. A pro-flux composition for coating ferrous metals with a thincoating of nickel prior to galvanizing comprising an aqueous solution ofless than 35% by Weight nickelous chloride, at lea-st 5% by weight of asubstance having a free aldehydic group, and at least 5% by weight of atleast one soluble halide salt selected from the group consisting ofammonium halides, alkali metal halides and alkaline earth metal halides.

11. A pre-flux composition according to claim 10 wherein the substancehaving a free aldehydic group is at least one selected from the group ofaldehydes, aldoses and carbohydrates and wherein the halide salt is achloride.

12. A pre-fiux composition according to claim 11 wherein the chloridesalt is sodium chloride.

13. An aqueous solution of from 5 to 35 by weight of nickelous chloridehexahydrate, from 5 to 10% by weight sodium chloride and from 5 to 30%by weight (References on following page) 8 References Clted by theExammer OTHER REFERENCES I UNITED STATES PATENTS I Wein: Gold Films,part 2, The Glass Industry, June 12,191,813 2/40 Brown 117-130 X 959,2,315,740 4/43 Schoonmaker et a1. 20440 X 5 2,762,723 9/56 Talmey et aLWILLIAM D. MARTIN, Pnmary Examzner. 2,940,870 6/ 6O Baldwin 11752 MURRAYKATZ, Examiner.

2,999,770 9/61 Gutzeit 1O6-1 X 3,015,858 1/62 Hendricks 11735 X

8. A METHOD OF GALVANIZING A FERROUS METAL COMPRISING PICKLING SAIDFERROUS METAL, PASSING THE PICKLED METAL THROUGH AN AQUEOUS PRE-FLUXSOLUTION COMPRISING AN AQUEOUS SOLUTION OF LESS THAN 35% BY WEIGHT OFNICKELOUS CHLORIDE HEXAHYDRATE, AT LEAST 5% BY WEIGHT OF A SUBSTANCEHAVING A FREE ALDEHYDIC GROP, AND AT LEAST 5% BY WEIGHT OF AT LEAST ONESOLUBLE HALIEDE SALT SELECTED FROM THE GROUP CONSISTING OF AMMONIUMHALIDES, ALKALI METAL HALIDE AND ALKALINE EARTH METAL HALIDES, ANDSUBSEQUENTLY PASSING THE PRE-FLUXED METAL THROGH A GALVANIZING BATH.